As many as 900 street parking spaces - one of San Francisco's most precious commodities - will be reserved for car-sharing vehicles and leased at discounted rates. The parking program, which will begin in the summer, is a two-year experiment that aims to spread car sharing throughout the city.

The San Francisco Municipal Transportation Agency approved the plan to set aside some of the city's 281,000 street parking spaces last year but still had to solicit interested companies and negotiate which parking spaces would be reserved for car-sharing vehicles. The agency approved the program after a smaller two-year test, involving a dozen street spaces, was deemed a success.

"Car sharing helps us achieve so many of our goals as a city," said Jay Primus, manager of SFpark, the city's parking management program. "It allows us to grow gracefully while still giving people access to a car."

The MTA recently selected three firms - national car-sharing company Zipcar, Bay Area nonprofit City CarShare and San Francisco's Getaround, which helps people rent out their own cars when they're not using them.

The city program will require that vehicles be available for sharing 75 percent of the time to make sure the spaces aren't being used for storage, and the companies must charge customers by the hour to keep traditional rental car operations from usurping the parking spaces.

Half of the street spaces - 450 - will be available starting in the summer. They'll be marked with signs and paint and assigned to a specific vehicle. Primus said they'll be spread around the city, which was a goal of the program.

While car sharing has taken off in San Francisco, most of the vehicles have been concentrated in downtown neighborhoods, and many users have been younger residents or people committed to living a relatively car-free existence.

Reaching outer districts

Because San Francisco is growing and the number of parking spaces is intentionally not being allowed to follow suit, the MTA has supported car sharing, and the agency, along with the Board of Supervisors, has strived to push car sharing out to more distant and less-affluent neighborhoods.

The street parking program used a combination of incentives and requirements to spread the spaces outside the central core. At least 30 percent of the spaces have to be in the outer two-thirds of the city, Primus said, and the price charged for spaces becomes less expensive in neighborhoods distant from downtown. The monthly fee ranges from $50 per space per month in the outer third of the city to $150 in closer-in neighborhoods to $225 in the downtown area.

"It's a big incentive to locate in the outer neighborhoods that haven't really been served by car sharing," he said.

The most popular neighborhood requested for spaces by the three companies is the Castro, Primus said. Its dense population, low rate of car ownership and shortage of parking spaces makes the program particularly attractive, he said.

Although the city has limited the number of reserved car-share spaces by all three companies combined to no more than two per block, Primus knows some residents will be distressed by what they see as a loss, or gift, of already scarce parking spaces to car-sharing operators.

"Anytime we change parking, there is opposition," he said. "The challenge for the MTA and the car-sharing companies is to get awareness of the benefits of car sharing. Academic research shows that every shared vehicle takes 10 private vehicles off the road. But we know that communicating that will be a challenge."

Convenient locations

Jessica Scorpio, founder of Getaround, said she's up for the challenge. Because her firm involves people leasing out their personal cars, they're not putting any additional cars on the street, she said, just making them more convenient for people to rent for short periods.

"We'll have cars all over the city - the whole 7 by 7," she said. "We're going to bring it to the Sunset, to the Bayview, to parts of the city that really haven't had car sharing."